This invention relates generally to the field of optical sensing, and more particularly to an optical assaying method and system having a moving sensor.
Extremely sensitive optical sensors have been constructed by exploiting an effect known as surface plasmon resonance (SPR). These sensors are capable of detecting the presence of a wide variety of materials in concentrations as low as picomoles per liter. SPR sensors have been constructed to detect many biomolecules including keyhole limpet hemocyanin, xcex1-fetoprotein, IgE, IgG, bovine and human serum albumin, glucose, urea, avidin, lectin, DNA, RNA, HIV antibodies, human transferrin, and chymotrypsinogen. Additionally, SPR sensors have been built which detect chemicals such as polyazulene and nitrobenzenes and various gases such as halothane, trichloroethane and carbon tetrachloride.
An SPR sensor is constructed by sensitizing a surface of a substrate to a specific substance. Typically, the surface of the substrate is coated with a thin film of metal such as silver, gold or aluminum. Next, a monomolecular layer of sensitizing material, such as complementary antigens, is covalently bonded to the surface of the thin film. In this manner, the thin film is capable of interacting with a predetermined chemical, biochemical or biological substance. When an SPR sensor is exposed to a sample that includes the targeted substance, the substance attaches to the sensitizing material and changes the effective index of refraction at the surface of the sensor. Detection of the targeted substance is accomplished by observing the optical properties of the surface of the SPR sensor.
The most common SPR sensor involves exposing the surface of the sensor to a light beam through a glass prism. At a specific angle of incidence, known as the resonance angle, a component of the light beam""s wavevector in the plane of the sensor surface matches a wavevector of a surface plasmon in the thin film, resulting in very efficient energy transfer and excitation of the surface plasmon in the thin film. As a result, at the resonance angle the amount of reflected light from the surface of the sensor changes. Typically, an anomaly, such as a sharp attenuation or amplification, is exhibited by the reflected light and the resonance angle of an SPR sensor can be readily detected. When the targeted substance attaches to the surface of the sensor, a shift in the resonance angle occurs due to the change in the refractive index at the surface of the sensor. A quantitative measure of the concentration of the targeted substance can be calculated according to the magnitude of shift in the resonance angle.
SPR sensors have also been constructed using metallized diffraction gratings instead of prisms. For SPR grating sensors, resonance occurs when a component of the incident light polarization is perpendicular to the groove direction of the grating and the angle of incidence is appropriate for energy transfer and excitation of the thin metal film. As with prism-based sensors, a change in the amount of light reflected is observed when the angle of incidence equals the resonance angle. Previous SPR grating sensors have incorporated square-wave or sinusoidal groove profiles.
Another highly-sensitive sensor that has been recently developed is known as a xe2x80x9cdiffraction anomalyxe2x80x9d sensor. Diffraction anomaly sensors include a substrate and a thin metal layer which are substantially the same as in an SPR grating sensor. In a diffraction anomaly sensor, however, a dielectric layer is formed outwardly from the metal layer and protects the metal layer from oxidation and general degradation. Typically, a sensitizing layer is formed outwardly from the dielectric layer. Diffraction anomaly sensors, like SPR sensors, exhibit a change in reflectivity, referred to as a diffraction anomaly, when exposed with a light beam at a particular angle of incidence. Unlike conventional SPR sensors, diffraction anomaly sensors exhibit a change in reflectivity for light polarized parallel to the grooves of the substrate. When a light beam has an angle of incidence equal to the diffraction anomaly angle for the sensor, the diffracted light beam propagates within the dielectric layer. In this manner, the dielectric layer acts as a waveguide and a change in reflectivity is readily detected by the controller. The diffraction anomaly is directly affected by the thickness of the dielectric layer. The effective index of refraction at the surface of the diffraction anomaly sensor changes in a manner similar to an SPR sensor when the diffraction anomaly sensor is smeared with a sample containing the targeted substance. Furthermore, the change in the diffraction anomaly angle is strongly dependent upon the amount of targeted substance present in the sample. In this manner, the diffraction anomaly sensor exhibits a shift in the anomaly angle that is comparable to an SPR sensor, even though the metal grating of the diffraction anomaly sensor is coated with a dielectric layer. Therefore, a quantitative measure of the targeted substance can be calculated by measuring the resulting shift in the anomaly angle.
In addition to individual sensors, there is considerable commercial interest in multiple-sensor systems that are capable of detecting a variety of targeted substances, such as certain odors, vapors, gases and other chemical species, in a surrounding environment or sample. By utilizing several sensors, such sensing systems are capable of simultaneously detecting several targeted substances. Other multiple-sensor systems utilize multiple sensors to recognize the presence of a single targeted substance. In this configuration, the burden of recognition does not lie upon a single sensor, but rests on the sensing system""s ability to properly interpret and recognize output patterns of the multiple sensors. Due to the use of multiple sensors, conventional multiple-sensor sensing systems are typically extremely expensive. Furthermore, conventional sensing systems are inherently complicated and therefore are not very portable.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an inexpensive, disposable sensing system which can assay a variety of substances in a sample. There is also a need for such a system that is compact, easy to manufacture and readily transported.
As explained in detail below, the present invention is directed to an optical assaying method and system having a movable sensor. In one aspect, the invention is a system for sensing a plurality of substances. The system includes a sensor having a plurality of sensitized regions, wherein each region is sensitized to at least one of the substances. A detector is responsive to light received from the sensitized regions of the sensor. A motor is coupled to the sensor for moving the sensor such that each sensitized region moves proximate to the detector. A controller is coupled to the detector for calculating a measure of at least one substance as a function of a detected change in light received from the sensitized regions of the sensor.
According to one aspect of the invention, the sensor of the sensing system is a rotating sensing disk driven by the motor. For example, in one embodiment the sensing system includes a sensor disk having a substrate coated with a plurality of indicator dyes sensitized to the plurality of substances. In this embodiment, the detector is responsive to spectral changes in light received from one or more of the indicator dyes.
In another embodiment the sensor of the sensing system is a constant grating sensor disk having a grooved substrate and a metal layer formed outwardly from the substrate. Furthermore, a dielectric layer is formed outwardly from the metal layer for suppressing reflection of incident light having a polarization parallel to the grooves of the substrate. In this embodiment, the dielectric layer continuously varies in thickness around the circumference of the sensor disk. The controller determines the measure of the substance according to a change in a position around the circumference of the sensor disk at which light received from the sensor disk exhibits an anomaly. The constant grating sensor disk may have a plurality of concentric grooves or may have a single groove spiraling from a center of the sensor disk to an outside edge of the sensor disk.
In another embodiment, the sensor of the sensing system is a radial grating sensor disk comprising a substrate having a plurality of grooves extending radially from a center of a surface of the substrate. Furthermore, a metal layer is formed outwardly from the surface of the substrate and substantially conforms to the grooved surface of the substrate. In this embodiment, the radially extending grooves may have a fixed period around a circumference of the sensor disk. As such, a light source scans the light beam radially across the surface of the rotating sensor disk and the controller determines a position along a radius of the sensor disk for each sensitized region at which detected light received from each sensitized region exhibits an anomaly. Alternatively, the radially extending grooves may have a period that varies around the circumference of the sensor disk and the light source exposes the sensor disk with the light beam at a fixed radius from the center of the rotating sensor disk.
In yet another embodiment, the motor does not rotate the sensor but linearly translates the sensor along a length of the sensor. In this embodiment, the sensor includes a substrate having a grooved surface such that the grooves of the surface have a period that varies along the length of the sensor.
According to one feature of the present invention, the various embodiments of the sensor may be a diffraction anomaly sensor that is responsive to a change in light having a polarization parallel to the grooves of the substrate. Additionally, the sensor of the present invention may be a surface plasmon resonance sensor responsive to a change in light having a polarization perpendicular to the grooves of the substrate.
According to another aspect, the invention is a method for assaying a substance in a sample including the step of providing a sensor disk having a metal diffraction grating having at least one groove, wherein the metal diffraction is coated with a dielectric layer having a thickness varying from a minimum thickness to a maximum thickness. The sensor disk is exposed with a light beam having a polarization component parallel to the grooves of the grating. The sensor disk is interacted with the sample and rotated. A measure of the substance in the sample is determined as a function of a shift in an anomaly position around a circumference of the sensor disk at which light received from the sensor disk exhibits an anomaly.
In yet another aspect, the invention is a method for assaying a substance in a sample including the step of providing a sensor disk having a metal diffraction grating having a plurality of grooves extending radially from a center of the sensor disk to an outer edge of the sensor disk. The sensor disk is interacted with the sample and rotated. A measure of the substance in the sample is determined as a function of a shift in an anomaly position along a radius of the sensor disk at which light received from the sensor disk exhibits an anomaly.
These and other features and advantages of the invention will become apparent from the following description of the preferred embodiments of the invention.